Hello everyone. How are you doing?
This article will serve as somewhat a continuation to our previous blog. In last blog, I described about out respiratory tract. This time we will start our discussion with Lungs.
Lungs are our destination factory where actual work is done after reaching via road i.e. respiratory tract.
After reading the article, you will be able to answer:
- Lungs structure and lobes.
- What are segments?
- What is pleura?
- How amniotic fluid is produced?
- What is oligohydramnios and how serious it is?
- Lungs anatomical position.
As you know, we have 2 lungs- Right and left. Left lung is smaller than the right one(Q1). Each lung has few lobes.
- Right lung has 3 lobes- superior, middle and inferior. Left has only 2- superior and inferior.
- There are 2 fissures in right lung i.e. horizontal fissure between superior and middle lobe, oblique between middle and inferior lobe. Left lung has only 1 fissure i.e. oblique between superior and inferior lobe.
- Each lobe has few segments.
Question for you 1. Can you answer, why left lung is smaller?
Ans 1. Pretty simple. Tell me, what else is present on left side of our chest. Yes, heart. As heart also shares some left space, left lung size is reduced than right lung. This is also the reason for why it has only 2 lobes. Though, it has lingula which can be considered as its 3rd lobe.
What are segments?
We will understand segments now. But like everywhere, whenever you enter somewhere you pass through a door. Similar to that, lungs also need a door where its contents could enter and exit, and that door is ‘Hilum’.
- Hilum is triangular space that is found on the medial(facing median of body) surface of the lungs.
- It contains, well, bronchi of course, but also vasculature serving both lungs and its bronchi.
- Pulmonary artery (Q2)- Enters into lungs, originate from right ventricle of heart and carries deoxygenated (dO2) blood to lungs.
- Pulmonary vein(A1)- After air exchange inside lungs, carry oxygenated (O2) blood to left atrium of heart.
- Bronchial arteries carry oxygenated blood for lungs.(Of course, lungs need their oxygen too.)
Q2. What is difference between pulmonary artery and artery?
A2. As we mentioned, pulmonary artery carries dO2 blood, which is not a common thing for artery. Arteries are normally known as to carry O2 blood. Because most of the time we speak in context of system i.e. body, for which artery carries blood from left ventricle of heart to system and veins carry dO2 blood from system to back to heart where again pulmonary artery carries it further. Blood circulation from left ventricle to right atrium is called systemic circulation and from right ventricle to left atrium is pulmonary circulation.
About1. Pulmonary vein(PV)- There are 4 pulmonary veins. 2 from each lung that enters into left atrium.
- Right superior PV- Drains upper and middle lobes of right lung
- Left superior PV- Drains upper of left lung
- Right inferior PV- Drains lower lobe of right lung
- Left inferior PV- Drains lower lobe of left lung
Right lung- has 3 lobes and 10 segments.
- Upper lobe- Apical, posterior and anterior.
- Middle lobe- Lateral and medial.
- Lower lobe- Superior, anterior, medial, lateral and posterior.
Left lung- has 2 lobes and 8 segments (some consider 10 segments by separating apicoposterior and anteromedial segments).
- Upper lobe- Apicoposterior, anterior, superiorlingular, inferiorlingular.
- Lower lobe- superior, anteromedial basal, lateral basal, posterior basal.
Significance of segments:
- Each segment has its own bronchus and arterial branch.
- Normally infections get restricted to one segmented as functionally segments are separated from one other.
- It also means that segments, if needed, could be separated without affecting other segments.
What is pleura?
Like segments, lobes are distinct too from each other. These lobes are divided by fissures which are actually formed by pleura.
But my question here is, what pleura?
Means, there are 2 parts of pleura i.e. Visceral and parietal.
We will answer that after knowing basic information about pleura.
A pleura is a serous membrane structure that forms inner layer i.e. visceral pleura and outer layer i.e. parietal pleura by folding on itself making a double layer.
- Visceral means layer that is related to viscera i.e. soft internal organs like lungs, heart etc.
- Covers outside of lungs and structure associated with it- vessels, bronchi and nerves too.
- As I said, it stretches into fissures and divides lung into lobes.
- Parietal means wall of body.
- Covers internal surface of thoracic cavity and superior surface of diaphragm (dome shaped muscle that has significant role in breathing).
- Forms pleural fluid and also reabsorbs it into lymphatics via stomata (openings through which pleural cavity and lymph vessels are connected).
- According to structure they are in contact with, they are divided into
- Cervical pleura- pleura extends slightly towards neck.
- Mediastinal pleura- covers mediastinum(A2).
- Costal pleura- covers inner surface of ribs, cartilages and muscles.
- Diaphragmatic pleura- covers thoracic surface of diaphragm.
A2: Mediastinum- A tiny but essential region that is present between lungs and holds heart, aorta, esophagus, trachea, thymus, nerves etc.
Both layers merges at hilum of each lung.
Now, after learning about both the layers, what do you think is present in between?
Yes, pleural cavity in which pleural fluid is present. It is:
- A potential space i.e. both layers, normally, are pushed on each other.
- Space for pleural fluid. Pleural fluid:
- Is pale yellow colored fluid that contains macrophages, lymphocytes, neutrophils and glucose.
- Creates tension between both layers so that during breathing both are pulled together.
Lubricates the surfaces between both layers so that they can slide on each other without any stress. Means, provide easiness for lung movements.
Q3. What happens when pleural cavity is damaged?
A3. Damage to pleural cavity can lead to pneumothorax i.e. leaking of air into cavity leading to surface tension loss. This causes collapsing of lungs as air presses against it.
Types: According to damage, it has few subtypes, from which important ones are mentioned here.
- Primary spontaneous-
- Collapsed lungs occur without any previous lung injury or disease.
- Most likely occur due to blebs. These are small air blisters that form and bursts that leads to leaking of air into cavities.
- Secondary spontaneous-
- Any existing disease that leads to collapsed lung.
- Eg, cystic fibrosis which is an inherited disorder that leads to damage of many digestive organs and lungs. Emphysema (one of the condition caused by Chronic obstructive pulmonary disease or COPD) where alveoli are damaged.
- Caused by physical damage like puncture wound, accident, piercing by knife or bullet.
- Person feels sharp pain in chest while breathing(could be unilateral)
- Take short (dyspnea) and fast (tachypnea) breathes
- Unable to breathing leads to reduced oxygen level hence bluish skin and tiredness
To diagnose: X-ray is performed where a shadow is seen without lung markings indicating collapsing.
- Bed rest immediately.
- Most commonly, thoracostomy is used. In this, a tube is inserted into the pleural cavity and excess air (or liquid) is removed (via suction device if needed).
- Oxygen therapy.
- If air patch is small, no treatment is needed as it can self heal.
What is oligohydramnios?
As we have discussed one of the problem of lungs, why not discuss another one which can lead to serious complications.
We have talked about it a little in our last blog.
Oligohydramnios- Just like always, we will break it into its roots i.e. ‘oligo’ and ‘hydramnios’.
Oligo means ‘few’ and hydramnios mean ‘building up of amniotic fluid’. Combining them we get, when the amniotic fluid is less than actual amount. (Though, hydramnios simply called as polyhydramnios means when amniotic fluid is excess.)
Normally it is 5 to 25 cm of AFI (amniotic fluid index, see A3) where below and above the range will lead to oligohydramnios and polyhydramnios respectively.
A3. AFI- It is an estimate of amniotic fluid where fetal well-being is monitored in biophysical profile(BPP). BPP- A scoring system of fetal health where they are evaluated by ultrasound in prenatal stage.
Before going to how oligohydramnios is caused, we have to understand…
How amniotic fluid is produced?
Amniotic fluid(AF) production starts just as gestation start and its growth precedes the growth of fetus as it has very supportive role for it.
Initially, the fluid comes from mother’s plasma. AF increases in outrageous manner as in 1st 10 weeks it is nearly 25 ml which within 5 weeks increases 4 folds and after 20 weeks, it becomes around 400 ml.
At this stage, its composition is similar to fetal plasma.
Simultaneously, after 5 weeks of gestation, fetal kidneys start producing urine. Also, fetal swallowing starts to function but both these has no function whatsoever to produce AF, atleast unless keratinisation starts.
Keratinization, a process where epidermal keratinocytes are differentiated by changes (formation of kertain polypeptides and polymerisation of kertain intermediate filaments i.e. tonofilaments) in their cytoplasm. It starts at 9th gestation week and completes after 25th week.
This keratinization makes fetal skin’s outer layer waterproof.
By 28 weeks, AF peaks at 800 ml which declines later at 400 ml at 42 weeks.
Now, keratinization checkpoints its symbolic changes in its composition. The composition is changed by fetal circulation of fluids like urine, secretions from mouth, nose and pulmonary fluid.
This composition keeps on changing with the demand of fetal development.
Contents and function:
As I mentioned, initially it is like water containing electrolytes mainly formed from mother’s body.
Later, when urine is formed, the composition is changed completely while fetus swallows and excretes it.
- It contains nutrients like proteins, carbohydrates, lipids, electrolytes, amino acids like glutamine (a precursor of nucleic acid biosynthesis), arginine ( for fetal and placental development) and hormones and enzymes, which are swallowed and excreted. Because of these, you can understand how it nourishes the baby altogether.
- Some growth factors like insulin-like growth factor I(IGF-I) for bone and tissue growth and transforming GF- alpha and beta 1 are also found.
- Provides innate immunity as it contains lysozymes, antimicrobials etc.
Causes of oligohydramnios:
- Poor formation of AF
- Poor fetal growth, that again leading to poor formation of AF
- Renal agenesis i.e. failure of fetal kidney to develop
- Twin-to-twin syndrome, where twins share placenta and blood vessels. In this, one fetus may send more blood to other leading to development of oligohydramnios in former and polyhydramnios in latter.
Oligohydramnios can lead to number of complications, in which the important one is Potter sequence where without proper amniotic fluid, fetus is compressed leading to structural deformities and death of baby.
Lastly I will mention one thing that will complete our blog. And that is, lungs position in our chest cavity.
You may find it interesting to know that the apex (upper edge) of lungs is actually above the 1st rib. The hilum is found at 5th-7th thoracic vertebral (T5-T7) level. Its base is found above diaphragm which is dome shaped and is stretched from T10-T12.
There were still a lot to mention but I think it is enough.
May be next time, I would mention some diseases related to it or may be something else. Whatever it will be, it will be informative and interesting. Later.